1. Field of the Invention
This invention relates to a pneumatic safety tire with excellent long-running ability after puncture.
2. Description of the Prior Art
A pneumatic safety tire is designed to ensure continuous safe running after puncture (to be referred to as "run-flat" condition, hereinafter) until reaching a place where replacement or repair of the punctured tire in a reliable and secure fashion. This occurs by having a means built in the tire for bearing the wheel load acting on the tire instead of bearing it by internal pneumatic pressure of the tire, which internal pneumatic pressure is normally maintained in the inner hollow space of the tire at a predetermined level but lost upon sudden occurrence of the puncture. What is meant by the "wheel load" is the total load applied to one third wheel from a vehicle to which the tire is mounted. Various types of the means for bearing the wheel load in case of tire puncture are known; for instance, U.S. Pat. No. 3,954,131 granted to BRIDGESTONE TIRE COMPANY LIMITED on May 4, 1976 teaches rubber reinforcements of sidewalls which provide run-flat condition ability without deteriorating regular performance before the puncture, especially without requiring any reduction of normal running speed.
In short, the aforesaid means of the prior art for bearing the wheel load upon occurrence of a puncture uses rubber reinforcements of crescent cross sectional shape made of an elastomer having a comparatively high elasticity and mounted primarily to the inner surface of tire sidewall portions to extend from bead portions to hump portions of the tire, the maximum thickness of each rubber reinforcement being less than 15%, preferably 3% to 9% of the maximum tire section width under inflated condition. The rubber reinforcements provide extra rigidity to the tire sidewall portions, for bearing the wheel load after tire puncture by the thus reinforced tire sidewalls instead of bearing the wheel load by the internal pneumatic pressure of the tire.
The aforesaid safety tire of the prior art, however, has a shortcoming in that it is difficult to determine criteria for setting an upper limit of the distance within which the run-flat condition is allowed while ensuring reproduction of the proper performance characteristics inherent to the tire by repairing the puncture. This is because the required distance to be covered by the run-flat is generally unknown and the time when the puncture is recognized is also uncertain. Accordingly, there is a tendency of using an excessive run-flat condition leading to breakage of the tire, or undertaking dangerous tire change operations on a road because of fear of tire breakage. Thus, the function of the safety tire has not been fully utilized.